We are studying the regulation of expression of the immunoglobulin gene family by attempting to answer two questions: (1) why do only cells of the B-lymphoid lineage synthesize immunoglobulins, (2) how do these cells transcribe only one or a few immunoglobulin genes, while leaving hundreds of other, similar immunoglobulin genes inactive? Our approach to these questions is to insert a cloned, rearranged kappa light chain gene into a plasmid in various configurations, to transfect the plasmid various type of cells, and to determine whether the transfected gene is transcribed. We have shown that the complete kappa gene is transcribed after transfection into antibody-producing myeloma cells but not in non-lymphoid 3T3 or L cells. Hence the different cell types are able to appropriate regulate the kappa gene even when not in its usual chromosomal environment. By deleteing different parts of the cloned gene, we have shown that certain sequence elements actually downstream of the promoter are necessary for its transcription in myeloma cells. We have localized the down-stream element to a 200 base pair region of DNA and have shown that it is an enhancer. We have recently base pair region of DNA and have shown that it is an enhancer. We have recently shown that both the enhancer and the promoter are cell-type specific, that is, they function in lymphoid cells but not in non-lymphoid cells.